Sulfuric acid is a basic chemical for various chemical industries and has been produced on a large scale for many years. Methods for producing sulfuric acid are broadly classified into two processes: lead chamber process and contact process. The lead chamber process includes steps of introducing sulfurous acid gas (SO2), which is obtained by roasting of metal sulfides such as sulfide ore, into a Glover tower and a lead chamber together with air, and allowing the sulfurous acid gas to react with air to produce sulfuric acid in the presence of a catalyst such as nitrogen oxide or nitric acid. On the other hand, the contact process, as described in Non-Patent Literature 1, includes steps of burning sulfur or roasting sulfides to produce sulfurous acid gas, allowing the sulfurous acid gas to proceed oxidation reaction for producing sulfur trioxide gas (sulfuric anhydride gas (SO3)) in the presence of a vanadium pentoxide (V2O5) catalyst, and allowing the sulfur trioxide gas to be absorbed in an aqueous sulfuric acid solution in an SO3 absorption tower to produce concentrated sulfuric acid.
However, the lead chamber process may cause coloration of sulfuric acid due to a residue of sulfuric acid hydrogen (nitrogen oxide) (HSO4.NO) produced as an intermediate product, and moreover this process can only produce a sulfuric acid with relatively low concentration. Therefore, in recent years, the contact process has been exclusively used which can efficiently produce concentrated sulfuric acid. It is to be noted that sulfuric acid hydrogen (nitrogen oxide) is referred to as nitrosylsulfuric acid, nitrosyl sulfate, or nitrosyl hydrogen sulfate (hereinafter, referred to as nitrosylsulfuric acid). The contact process may also contain nitrosylsulfuric acid in the concentrated sulfuric acid when nitrogen oxide, produced by the roasting treatment or the like of sulfides, coexists in the treated gas. However, since the concentration of the nitrosylsulfuric acid in this case is low, there is almost no problem with the quality of the concentrated sulfuric acid as a product.
Recently there is a trend to emphasize so-called eco-friendly facilities which give consideration for environmental conservation. In response to this trend, various measures are also implemented in sulfuric acid production facilities. For example, the above-stated SO3 absorption tower is designed to have two-stage configuration so as to improve a removal rate, and packing material for the SO3 absorption tower is improved so as to enhance a gas-liquid contact efficiency. Studies on the improvement of the catalyst and optimization of the operating temperature are also made to improve an oxidation rate of sulfurous acid gas, so as to decrease a residue of sulfurous acid gas that is left unabsorbed in the SO3 absorption tower while enhancing the operating efficiency of the sulfuric acid production device.
Further, a desulfurization device is provided downstream of the SO3 absorption tower such that sulfur oxides such as sulfurous acid gas and SO3, which are generated in the sulfuric acid production step, are removed to a lower concentration level. An example of such a desulfurization device is a desulfurization tower 2 as shown in FIG. 1 which is conventionally provided to desulfurize the outlet gas from a sulfuric acid production device 1 consisting of an SO3 oxidation tower 1a and an SO3 absorption tower 1b, and an alkali neutralization method using NaOH or Mg(OH)2 or a lime-gypsum method using limestone to produce gypsum as a by-product is performed in the desulfurization tower 2. However, this method requires a chemical such as neutralizing alkali and generates sulfate as a waste, which necessitates complicated operation such as handling of solids and complicated operational control for the use of the chemical and dispose of the waste. Accordingly, this method imposes a large cost and moreover has a concern to cause secondary pollution.
Under the circumstances, there is proposed a method for desulfurizing a flue gas using an activated carbon instead of the above-stated method. For example, Patent Literatures 1 and 2 propose a method in which a flue gas containing sulfur oxide is brought into contact with a catalyst to convert the sulfur oxide to dilute sulfuric acid, and then the dilute sulfuric acid is collected and removed. The catalyst used in this method is produced by adding fluororesin to an activated carbon power, kneading them under application of a shear force, and then forming the kneaded product into a predetermined shape.
Patent Literature 3 discloses a flue-gas-desulfurizing method in which a flue gas containing sulfurous acid gas is processed by two stages where sulfurous acid gas in the flue gas is absorbed by a liquid absorbent of limestone slurry in the first stage, and a residue of the sulfurous acid gas is processed with an activated carbon catalyst in the second stage. A dilute sulfuric acid produced in the second stage is disposed of by being mixed with the liquid absorbent in the first stage.
Patent Literature 4 proposes a technique in which a purification tower is provided downstream of a flue gas desulfurization device that removes sulfur oxides in a flue gas. Patent Literature 4 discloses that a catalyst layer made of activated carbon fibers is provided in the purification tower, and that the flue gas treated in the flue gas desulfurization device is introduced to the catalyst layer together with water so as to produce dilute sulfuric acid from sulfur oxides. It also discloses that the dilute sulfuric acid thus produced is supplied to the flue gas desulfurization device and is disposed of therein.
Patent Literature 5 discloses a flue-gas-desulfurizing method in which a flue gas containing sulfur oxides is humidified and cooled, and then introduced to an activated carbon-based catalyst to produce dilute sulfuric acid from the sulfur oxides in the flue gas. Patent Literature 5 also discloses a technique in which the dilute sulfuric acid is condensed by using heat of the flue gas.